1. Field of the Invention
The present invention relates to a radiation imaging apparatus that obtains radiation images and a dark current correction method for the apparatus.
2. Description of the Related Art
In obtaining radiation images (for example, X-ray images), an X-ray image of an object is acquired by using a film/screen with a cassette holding a film and an intensifying screen or an image plate in a cassette which is used for computed radiography.
Recently, X-ray detectors that directly convert X-ray images into digital signals in real time have been proposed. Such X-ray detectors include, for example, an X-ray detector having a solid-state photodetector and a scintillator stacked on each other. The solid-state photodetector has solid-state photodetection elements, each formed from a transparent conductive film and a conductive film, arrayed in a matrix form on an amorphous semiconductor on a substrate made of silica glass. The scintillator converts X-rays into visible light.
There are also various known X-ray detectors that directly acquire X-rays with solid-state photodetectors without using a scintillator. Such an X-ray detector is free from the influence of light scattering caused by a scintillator unlike an X-ray detector using a scintillator, and hence is generally considered to have a high resolution. An X-ray detector that includes a combination of a CCD or CMOS detector and a scintillator to increase the number of images taken per unit time, is also known.
In general, these X-ray detectors detect the intensities of X-rays as electric charge quantities. For this reason, in order to acquire an X-ray image, it is necessary to reset the electric charges in pixels, accumulate electric charges, transfer the electric charges in pixels, and perform driving in a predetermined cycle.
In an X-ray detector, the electric charges of dark current components proportional to the accumulation time of signal charges generated by X-rays are accumulated together with the signal charges. For this reason, an acquired X-ray image contains X-ray signal components and dark current components, and hence dark current correction processing is performed in X-ray imaging. In dark current correction processing, a dark current image containing only dark current components is acquired without X-ray irradiation. The acquired dark current image is then subtracted from the X-ray image to remove the dark current components from the X-ray image.
In this case, as described above, a dark current image is acquired without X-ray irradiation. For this reason, in general, when obtaining a still image, the X-ray detector acquires the image immediately before or after X-ray irradiation. When obtaining a moving image typified by an X-ray fluoroscopic image, since it is generally necessary to observe an X-ray image in real time, the X-ray detector acquires the image before or between times of X-ray irradiation. In high-speed imaging such as IVR, angiography, or CT, in which a larger number of images (for example, 60 fps) are acquired per unit time, it is difficult to acquire a dark current image between times of X-ray irradiation. For this reason, in such a case, the X-ray detector often uses a dark current image acquired before X-ray irradiation.
In general, however, in such an X-ray detector, dark current components are often unstable immediately after driving has started. In order to improve the quality of an obtained image, therefore, it is necessary to secure a certain period of time from the start of driving to X-ray irradiation. On the other hand, in order to improve the operability of an X-ray imaging apparatus, it is preferable to quickly start X-ray irradiation when the operator presses the start switch (for example, the X-ray irradiation switch).
To eliminate this tradeoff, Japanese Patent Laid-Open No. 07-236093 discloses a technique of changing and using dark current components stored in advance, in accordance with an imaging time, the temperature of a solid-stage imaging device, a pixel value, a pixel position, and the like. Japanese Patent Laid-Open No. 2007-222501 also discloses a technique of dividing the time between the driving starting (power being applied) and the pixel characteristics becoming stable into a plurality of intervals and measuring and storing dark current components in the respective intervals in advance. This technique performs correction by subtracting a dark current component corresponding to each interval at the time of imaging.
As in the case of the technique disclosed in Japanese Patent Laid-Open No. 07-236093, when dark current components stored in advance are to be changed and used in accordance with the situation at the time of imaging, it is necessary to monitor the situation at the time of imaging. This makes it necessary to provide a new arrangement. In addition, it is difficult to accurately change dark current components in accordance with all the situations that can occur.
As in the case of the technique disclosed in Japanese Patent Laid-Open No. 2007-222501, when dark current components after the start of driving are to be stored in the respective intervals, it is impossible to give sufficient consideration to errors due to actual situations at the time of imaging, for example, the imaging frame rate and the temperature of the solid-state imaging device.
A simple measure to eliminate the above tradeoff is to stabilize dark current components by always driving an X-ray detector, i.e., always energizing it. In this case, however, there is a fear that, for example, the power consumption will increase and the service life of the apparatus will shorten.